Papers by Keyword: Bake Hardening

Abstract: The effect of natural aging on mechanical properties and bake hardening behaviors of Al-Mg-Si alloys was quantitatively investigated by a series of tensile experiments along the rolling direction. The natural aging period is from three days to three months after heat treatment. As the results, within three months, along the rolling direction, 0.2% offset yield strength and ultimate tensile strength respectively increased from 97 MPa to 145 MPa, 210 MPa to 248 MPa. The strain hardening exponent n-value and the increment of yield strength after bake hardening respectively decreased from 0.2804 to 0.2186, 127 MPa to 89 MPa. The percentage elongation after fracture varies from 22% to 24% during natural aging and varies from 13% to 16% after bake hardening. A large amount of detailed data has been given, which quantitatively describes the change in mechanical properties and bake hardening behaviors of Al-Mg-Si alloys during natural aging.

Abstract: In this paper the strain hardening property and the bake hardening property of an original dual phase steel with high Aluminum had been studied. It can be disclosed that the dual phase steel has an excellent strain hardening property and bake hardening property with a strain hardening value of 125MPa and a bake hardening value of 60MPa as well as a total hardening value of 162MPa. The excellent properties of bake hardening and strain hardening will make some contribution to the application properties of the auto parts made by the new dual phase steel. Key Words:dual phase steel, strain hardening, bake hardening

Abstract: The grain boundary segregation of carbon atoms is influenced by grain size and annealing condition. Increasing grain size and improving the final cooling ( ＜400 oC) rate can improve the bake hardening property, due to the decreasing of carbon segregation at grain boundaries. Cooling rates have a great effect on bake hardening property of ULC steel with smaller grains

Abstract: The bake-hardening experiment was carried out for the Ultra-low carbon steel under different pre-strains, and the bake-hardening property is determined. The results are as follows: when the pre-strain is in the range of 0% ~4%, the BH value increases with the increase of the pre-strain; while in the range of 4% ~ 8%, the BH value decreases with the increase of the pre-strain, and when the pre-strain is 4% , the corresponding BH value is higher than others. When the pre-strain is from 8% to 15%, The BH value gets improved with the increase of the pre-strain, and the BH minimum value is corresponding to 8% pre-strain. While the results of Transmission electron microscopy(TEM) analysis shows that the dislocation density of 4% pre-strain is higher than that of 8% obviously, which improves that the dislocation density is the main factor that affects BH value.

Abstract: Effect of grain boundary on strain ageing behaviour of Nb-bearing ULC steel sheets has been studied at the aging temperature from 70 to 220°C, using 2% pre-strained specimens with different ferrite grain sizes of 9.5μm and 183μm. Two different hardening stages were exhibited in the fine-grain specimen, whereas only a single hardening stage was shown in the large-grain specimen. The increase in YP of the first hardening stage was around 30MPa; the activation energy of this stage was estimated to be from 83 to 86kJ/mol, which is close to that of body diffusion of carbon atoms in α-Fe. The increase in YP of the second hardening stage reached 90MPa; the activation energy was 135kJ/mol, which is close to that of body diffusion of Fe atoms in grain boundary and precipitation of η-carbide. From TEM observations and nanoindentation analyses, it was inferred that the dominant mechanism could be dislocation pinning by carbon atoms for the first hardening stage, and grain boundary hardening or hardening around it for the second.

Abstract: The increased demand for advanced high-strength steels (AHSS) is driven mainly by the need of the automotive industry to reduce weight and to improve safety. Beside good ductility and high strength, those steels have a high bake hardening (BH) effect, giving additional contribution to the strength of structural parts, subjected to the paint baking process. In this paper results are presented gained from hot rolled martenstitic steels. For the simulation of changing conditions within the final hot rolling process the specimens were hot deformed using different schedules changing reductions and temperatures been selected according to the non-recrystallization temperature By controlling the deformation temperature and the amount of strain below non-recrystallization during the thermomechanically controlled process it was possible to refine the martensitic steel structure, resulting in an improved strength and BH behaviour. A wide spectrum of mechanical properties could be obtained. The best strength and BH levels were recorded where the deformation was applied below T_nRX at the highest amount of strain.

Abstract: Effects of pre-straining, baking temperature and baking time on bake hardening (BH) behavior of cold-rolled dual phase steel were investigated. The results show that the investigated dual phase steel has clear BH behavior. Increased pre-straining led to the BH value decreased, while increased the baking temperature and prolonged baking holding time can all increase the BH value. The mechanics of bake hardening behavior in dual phase steel were discussed, and the effects of pre-staining on tensile strength after baking were also analyzed.

Abstract: Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.

Abstract: The investigations deal with processes leading to local effects of strengthening in multiphase steels, being characterized by good formability, continuous yielding, high strength and a strong bake hardening and ageing effect. Dual phase and complex phase steels are under investigation to examine the effect of thermo-mechanical processing parameters on local ageing ability and its use for designed properties. For this purpose local heat treatment by laser are studied, as well as stability of local ageing on the adjusted strength. A remarkable increase of the hardness in the heat treated zone was observed. Stability of the local strengthening effect could be confirmed. Partial heat treatment of multiphase steels by laser can open a new field of application for the local use of the strengthening effect to influence only relevant areas, thus giving potential for energy saving.

Abstract: The performance of multiphase steels with high strength and improved toughness or ductility, such as intercritically annealed dual-phase (DP) and transformation-induced plasticity (TRIP) steels, is of key importance to the automotive industry. In this work we have considered the entire manufacturing process and the effects of this on the final product performance. These steels are formed to produce the required final shape and then the car is paint baked. In this work we also consider the effect of cold working and bake hardening on the fatigue life of the components.